MODIFICATION OF THE GREAT LAKES
points on this line, called the isobase of the outlet, are raised or
lowered equally by the tilting and are unchanged with reference
to one another. All points southwest of it are lowered, the
amount varying with their distances from the line, and all points
to the northeast are raised. The water, always holding its sur
face level and always regulated in volume by the discharge at
the outlet, retreats from the rising northeast coasts and encroaches
on the sinking southwest coasts. Assuming the rate of tilting
to be 0.42 foot per 100 miles per century, the mean lake level is
rising at Duluth 6 inches per century and falling at Heron bay
5 inches. Where the isobase intersects the northwestern shore,
which happens to be at the international boundary, there is no
change.
Lake Ontario lies altogether southwest of the isobase of its
outlet, and the water is encroaching on all its shores. The same
tilting that enlarged it from the area markt by the dotted line of
figure 2 is still increasing its extent. The estimated vertical rise
at Hamilton is 6 inches per century. The whole coast of Lake
Erie also is being submerged, the estimated rate at Toledo and
Sandusky being 8 or 9 inches per century.
The isobase of the double Lake Huron-Michigan passes south
west of Lake Huron and crosses Lake Michigan. All coasts of
Lake Huron are therefore rising as compared to the outlet, and
the consequent apparent lowering of the mean water surface is
estimated at 6 inches per century for Mackinac and at 10 inches
for the mouth of the French river on Georgian bay. In Lake
Michigan the line of no change passes near Manistee, Michigan.
At Escanaba the estimated fall of the water is 4 inches per cen
tury; at Milwaukee the estimated rise is 5 or 6 inches, and at
Chicago between 9 and 10 inches.
These slow changes of mean water level are concealed from
ordinary observation by the more rapid and impressive changes
due to variations of volume, but they are worthy of considera
tion in the planning of engineering works of a permanent char
acter, and there is at least one place where their influence is of
moment to a large community. The city of Chicago is built on
a smooth plain little above the high-water level of Lake Michigan.
Every decade the mean level of the water is an inch higher, and
the margin of safety is so narrow that inches are valuable. Al
ready the older part of the city has lifted itself several feet to
secure better drainage, and the time will surely come when other
measures of protection are imperatively demanded.